At the heart of a color sensor is a photodetector. The purpose of the photodetector is to capture and convert electromagnetic radiation into an electronic signal. Typically, the resulting electronic signal is proportional to the incident light intensity. Conventional color sensor circuits employ p-i-n or pn junction photodiodes as the photodetector. In general, a photodiode captures and absorbs light energy. Charge carriers are produced and swept across the p-i-n or pn junctions of the photodiode and produce a small photocurrent, which can be detected. Normally, the photodiodes exhibit a linear response over a large operational range.
In the color sensor, a transimpedance amplifier is used to convert the photocurrent into a voltage signal. The transimpedance amplifier is beneficial as it provides a high effective input impedance to generate a suitable voltage level while maintaining a R-C time constant that is significantly lower than a conventional amplifier with similar input impedance characteristics. Moreover, the transimpedance amplifier also provides a linear response and does not compromise the inherent high linearity of the photodiode.
However, in the conventional color sensor, problems with dark current null offset and temperature variation occur that adversely affect the voltage response from the transimpedance amplifier of the color sensor. Prior Art FIG. 1A is a graph illustrating the effects of temperature variation and the dark color offset in a color sensor. Prior Art FIG. 1B is a graph illustrating a blown up region 100B of Prior Art FIG. 1A illustrating the dark current null offset.
Specifically, in a color sensor, dark current representing dark light energy contributes a voltage offset. As shown in Prior Art FIG. 1B, the dark current voltage offset is approximately 4 mV at zero amps for the detected photocurrent, in one case. This dark current voltage offset adversely affects the DC response of the color sensor. That is, the dark current voltage offset negatively affects the output voltage from the color sensor to give a false voltage reading corresponding to the intensity of the light energy input.
Moreover, the rate of the dark current voltage offset varies with temperature. Prior Art FIG. 1A is a graph illustrating the effects of the variation of the dark current voltage offset on the voltage response of the transimpedance amplifier of the color sensor. In particular, the dark current offset rate increases with increased temperatures. This change in rate of the dark current voltage offset affects the slope of linearity in the transimpedance amplifier response. That is, the slope of the line indicating operation of the color sensor at T=100 degrees Celsius is greater than the slope of the line indicating operation of the color sensor at T=25 degrees Celsius. Appropriately, the slope of the line indicating operation of the color sensor at T=25 degrees Celsius is greater than the slope of the line indicating operation of the color sensor at T=minus 40 degrees Celsius. As such, the color sensor is sensitive to temperature as the voltage reading from the color sensor does not provide a constant linear response over a range of temperatures, which adversely affects the response of the transimpedance amplifier in the color sensor.
As a result, because conventional color sensor amplifiers exhibit dark current null offset in their voltage outputs, a disadvantage of conventional color sensors is decreased DC response (zero offset) of the color sensor. In addition, another disadvantage is that temperature variation in the voltage response of the transimpedance amplifier of the color sensor contributes to a variation in the slope of the linear response that adversely leads to a variation in the voltage response of the color sensor over a temperature range.
Therefore, prior art systems and methods provide color sensors that are adversely affected by dark current voltage offset and reduced temperature robustness.